(1) Field of the Invention
The present invention relates to technology for monitoring the quality of wavelength division multiplexed signal light transmitted in optical communications, and more particularly, relates to a method and apparatus for monitoring deterioration of signal quality based on a state of occurrence of bit error and an optical transmission system using the same.
(2) Related Art
Heretofore, in a long distance optical transmission system, an optical signal has been converted into an electrical signal to be transmitted using regenerative repeaters which perform reshaping, retiming, and regenerating. At the present time, however, with the development of optical devices including optical amplifiers and the development of transmission technology including the wavelength division multiplexing (WDM) optical transmission system, it is possible to transmit optical signals of large capacity over a distance of several thousand kilometers without converting them into electrical signals. In addition, in next generation optical transmission networks, it is expected that, by introducing not only optical amplification repeater nodes but also optical add/drop multiplexer (OADM) nodes, optical cross-connect (OXC) nodes, hub (HUB) nodes and the like, photonic networking of mesh topology will be performed.
FIG. 13 is a diagram showing an example of configuration of a conventional optical transmission system using electrical regenerative repeaters. In the optical transmission system of FIG. 13, a plurality of optical transmission/reception sections 100 are connected with each other by an optical transmission path 101 on which regenerative repeaters 102 are arranged at required intervals. The most direct technique for monitoring signal quality in such a conventional optical transmission system is a method of monitoring a bit error rate (BER) at an electrical stage when performing the electrical regenerative repeating in each regenerative repeater 102. Specifically, in SONET/SDH system optical transmission networks currently in practical use, it is possible to estimate the bit error rate utilizing an error detection bit (i.e., B1, B2 byte) in the overhead of transmission data and therefore, it is possible to obtain an amount of deterioration of signal quality at each link connecting optical transmission/reception sections 100. In such a network, if the signal quality is significantly deteriorated, a measure to switch the path or the like is adopted. If the deterioration is in an allowable range, a simple measure to adjust the signal light power or the like is adopted. Further, in a system adopting error correction technology, it is also possible to measure the frequency of error corrections to estimate the bit error rate. Further, a method of estimating the Q-value from a signal light waveform is also known.
Other than the above methods requiring the measurement of the bit error rate, for example, as shown in FIG. 14, there is a method in which an optical branching device 106 is provided on an optical transmission path 105 between an optical transmission section 103 and an optical reception section 104, the signal quality is monitored by extracting a part of WDM optical signal to measure the optical spectrum of the extracted optical signal by an optical spectrum analyzer 107. With this method, the signal quality is estimated based on the optical power level of the optical signal at each wavelength.
Further, as described above, at the present, the long distance transmission is possible without converting an optical signal into an electrical signal. Examples of configuration of such an optical transmission system are shown in (a) to (c) of FIG. 15.
The system of (a) of FIG. 15 is a long distance transmission system using only optical amplification repeater nodes N1 as a plurality of repeater nodes existing on an optical transmission path 202 connecting an optical transmission section 200 and an optical reception section 201. A signal light is transmitted just as in a state of light from the optical transmission section 200 to the optical reception section 201. Each of the optical amplification repeater nodes N1 usually has an optical amplification function and a wavelength dispersion compensation function. Further, the system of (b) of FIG. 15 adopts a compensation node N2 in addition to the above optical amplification repeater nodes N1. If the WDM signal light is transmitted just as in the state of light over a long distance, depending on the accumulation of gain deviation in the optical amplifiers or wavelength dispersion, there occurs a channel in which a transmission characteristic required in the system cannot be satisfied. To suppress the occurrence of such a channel to enable the long distance transmission, as shown in (b) of FIG. 15, it becomes necessary to adopt the compensation node N2 having not only the optical amplification function and the dispersion compensation function, but also a gain equalization function, a dispersion slope compensation function or the like, corresponding to required compensation intervals. Further, as shown in (c) of FIG. 15, by using a plurality of compensation nodes N2, it becomes possible to realize an ultra-long distance optical transmission system.
Further, in the next generation optical transmission system as shown in FIG. 16 for example, in addition to the optical amplification repeater nodes N1 and compensation nodes N2, hub nodes N3 each having an optical path switching function are adopted to realize an optical network different from the conventional point-to-point transmission.
When the monitoring of signal quality is introduced to a system in which a WDM signal is transmitted for a long distance just as in the state of light as shown in FIG. 15 or FIG. 16, it is possible to use, for example, an optical branching device to extract a part of the WDM signal light at each node and utilize an optical spectrum analyzer to monitor the signal quality as shown in the above-mentioned FIG. 14, or to convert the extracted WDM signal light into an electrical signal, and then measure the bit error rate to monitor the signal quality (refer to Japanese Unexamined Patent Publication No. 8-321805 and Japanese Unexamined Patent Publication No. 2000-31900).
However, there are problems in the above conventional technology for monitoring the signal quality: (a) an increase of the time required for improvement of the signal quality and the complication of the process required for maintenance of the signal quality; and (b) the reduction in the measurement accuracy of the signal quality.
First, the above problem (a) will be described specifically. In general, as one method for setting a wavelength path (optical path) in a network, there is a method of determining, at the time of installing the network, the types of wavelength paths which can be set for combinations of a certain transmission node and a certain reception node, and when a request for setting a wavelength path is issued at the in-service time, selecting to use a wavelength path not being used among the wavelength paths determined at the time of installation. Another method is for actually transmitting a light of candidate wavelength in a candidate route when a request for setting a wavelength path is issued at the in-service time, and after confirming that the sufficient signal quality is ensured, transmitting signal light carrying actual data. The latter method has an advantage of enabling the construction of a more flexible network.
However, in a system using the latter method, if the wavelength spacing is narrowed or the signal light wavelength bandwidth is extended by increasing the wavelengths, there is a possibility that the quality of transmitted signal light is deteriorated, due to a nonlinear effect which never occurred in previous operation time, such as, cross-phase modulation (XPM), intra-channel four-wave mixing (IFWM), stimulated Raman scattering (SRS) or the like. When the signal quality no longer satisfies a value required by the system, the working becomes necessary for improving characteristics, such as, specifying a deterioration factor of the signal quality and then specifying the transmission block requiring countermeasures. In practice, a system manager has to achieve the improvement of the characteristic while repeating a trial and error process. Such working by the system manager has a high possibility of requiring a long time until the suitable setting is performed, causing a major demerit to a communication common carrier using the system.
Further, for example, in the case where a deterioration rate of the signal quality has been increased caused by the deterioration with age of system components such as the optical transmission path, it is desirable to quickly improve the characteristic using a simple method other than the switching of the wavelength path, while the signal quality still satisfies an allowable value of the system. One of the most effective methods is to adjust the optical power of each wavelength in the WDM signal light. However, in a system adopting optical add/drop multiplexer nodes, optical cross-connect nodes, or hub nodes, there is a possibility that since the quality deterioration factor of the signal light of each wavelength cannot be specified, the location where the power of the signal light is to be adjusted and a reset value of the signal light power will become unclear. Such a circumstance is particularly remarkable in a long distance transmission system and a network configuration system. Accordingly, there is caused a problem in that a long time is required until the suitable setting is performed or the process required for maintaining the signal quality is complicated.
Next, the above problem (b) will be described specifically. The conventional technology for monitoring the signal quality as described in the above requires a long time for measuring the bit error rate when directly measuring it. However, a relatively high speed process becomes possible by adopting a method of measuring the optical spectrum to investigate a signal light power to noise light power ratio (OSNR) or a method of estimating the signal quality from information of the signal light power of each channel. In monitoring by such optical spectrum measurement, however, high accurate measurement of the noise light power becomes difficult in a high density WDM transmission system with signal light wavelength spacing of 50 GHz or less. Therefore, it is difficult to expect accurate estimation of bit error rate, resulting in a problem in terms of measurement accuracy.